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For most new Python test suites, pytest is a flexible starting point: it supports plain assertions, automatic test discovery, reusable fixtures, and existing unittest tests. Choose Python’s built-in unittest when standard-library availability or an established class-based suite matters more. Use Selenium or Playwright when the test must interact with a real browser; they are browser-automation tools, not substitutes for choosing a unit-test runner.
The right setup depends on what the test needs to prove, the code and workflow already in place, browser coverage, and what your CI agents can install and run. No single framework is best for every Python project.
Choose a framework by test scope
| What you need to test | A practical starting point | Why |
|---|---|---|
| Functions, modules, or application logic without browser interaction | unittest or pytest |
unittest is included with Python; pytest offers functions, plain assertions, fixtures, and discovery. |
An existing unittest suite you want to extend |
Keep unittest, or introduce pytest incrementally |
pytest documents support for running unittest tests, so a wholesale rewrite is not required. |
| Interactions with a web page in a real browser | Selenium or Playwright, commonly run with pytest or unittest | These tools automate browsers. They complement a test runner rather than replace the role of one. |
Before choosing, consider the existing suite and test layout, the authoring style your team can maintain, which browsers and remote sessions matter, and whether CI can provide browser binaries and operating-system dependencies. These are decision factors, not benchmark criteria: the official documentation does not establish a universal speed or reliability winner.
When Python’s built-in unittest is enough
unittest ships with Python and provides test cases, fixtures, suites, runners, command-line execution, and test discovery. Tests commonly subclass unittest.TestCase; test methods conventionally begin with test. Setup and cleanup methods let a case prepare and release resources.
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import unittest
def total(prices):
return sum(prices)
class TotalTests(unittest.TestCase):
def test_adds_prices(self):
self.assertEqual(total([3, 4]), 7)
if __name__ == "__main__":
unittest.main()
Save this as test_totals.py and run it directly with python test_totals.py. For discovery across a project, run python -m unittest discover from the project directory. Use unittest when avoiding an external test-runner dependency is useful, or when the project already relies on its class-based conventions and named assertion methods. Python’s documentation describes test cases as runnable in isolation or in arbitrary combinations, which makes independent tests a sound goal for either framework.
When pytest is a good starting point
pytest is an external framework and test runner. Its documented features include plain assert statements with detailed failure introspection, automatic discovery, modular fixtures, and support for running unittest suites. The current pytest overview documents Python 3.10+ or PyPy 3 support; check its live compatibility page before adopting a version constraint, because compatibility can change.
Install and run a small suite
Keep test dependencies isolated in a virtual environment, as pytest’s good-practices guidance recommends. For example:
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python -m venv .venv
# Activate the environment for your shell, then:
python -m pip install pytest
python -m pytest
pytest conventionally discovers files named test_*.py or *_test.py. A minimal test file can use ordinary functions:
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# test_totals.py
def total(prices):
return sum(prices)
def test_adds_prices():
assert total([3, 4]) == 7
Run a specific file with python -m pytest test_totals.py. The assertion failure output includes the compared values, so tests can often use the language’s normal assertions rather than a separate assertion method.
Organize the project and install its package
pytest documents both separate test directories and other layouts. For a small project, tests beside the package may be straightforward; for a larger application, a dedicated tests/ directory can make the boundary clear. Keep discovery names consistent so the runner finds tests as intended. If the application itself is a package, pytest’s guidance describes editable installation during development; the exact installation command depends on the project’s packaging configuration.
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Adopt pytest without discarding unittest
pytest supports existing unittest.TestCase tests out of the box. A team can install pytest, run the current suite through it, and add new tests using pytest’s function-and-fixture style as needs arise. Migration can therefore be incremental rather than a rewrite. Retain a consistent discovery and setup approach, and migrate older tests only when there is a practical maintenance benefit.
Use Selenium or Playwright for browser behavior
Browser tests answer questions that function-level tests cannot, such as whether a user can complete a workflow through a rendered page. Selenium WebDriver and Playwright both automate browsers, and Selenium’s Python documentation demonstrates integration with both unittest and pytest. Choose based on the browsers, session model, existing test code, setup constraints, and debugging workflow you need; the available documentation does not establish that either tool is categorically faster or more reliable across all environments.
Selenium: manage the driver lifecycle
Selenium’s Python API documentation identifies version 4.50.0 and says modern Selenium uses Selenium Manager to handle browser and driver installation when a WebDriver is instantiated. Verify browser setup in the actual environment, especially CI. Always close the browser session, including when an assertion fails. The documented examples use cleanup or fixture teardown for driver.quit().
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import pytest
from selenium import webdriver
from selenium.webdriver.common.by import By
@pytest.fixture
def driver():
browser = webdriver.Chrome()
yield browser
browser.quit()
def test_homepage_title(driver):
driver.get("https://example.com")
assert "Example" in driver.title
This example assumes a Chrome browser can be launched in the environment. For remote WebDriver sessions, Selenium requires a Selenium Grid. Browser startup, network access, and application readiness should be accounted for in the CI environment and test design.
Playwright: prepare browsers and CI dependencies
Playwright’s Python CI guidance presents a sequence: make browsers available to the CI agent, install Playwright and browser dependencies, then run pytest. One documented installation pattern is:
python -m pip install pytest-playwright
playwright install --with-deps
python -m pytestUse the installation instructions appropriate to your operating system and CI image; the command is not a guarantee that every environment is configured identically. Playwright’s example GitHub Actions workflow retains traces on failure and uploads test artifacts. Those are useful debugging options, not universal requirements. Its examples include Python 3.13 and a v1.63.0 container, but those example versions should not be treated as a general compatibility promise.
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Keep browser tests focused and maintainable
Selenium’s project guidance emphasizes that browser complexity and cross-browser behavior make functional tests challenging, and that automating interactions does not by itself create a well-architected suite. The project states, “No one approach works for all situations,” and asks readers to apply its guidance to their environment. Keep browser tests targeted at important user-visible behavior; isolate setup and cleanup, and make failures diagnosable with the artifacts your CI can retain.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Practices that improve a Python test suite
- Make tests independent. Avoid relying on execution order or state left behind by another test. Use setup and cleanup deliberately so a test can run alone or alongside other cases.
- Isolate dependencies. Develop and run the suite in a virtual environment so project and test dependencies do not unintentionally affect the system Python installation.
- Use predictable discovery. Follow the runner’s naming conventions and choose a test layout appropriate to the project’s size.
- Reuse what works. Keep a functioning
unittestsuite when introducing pytest; use incremental adoption rather than rewriting solely to change runners. - Clean up browser sessions. Ensure teardown closes the driver even if a browser test fails.
- Make CI setup explicit. Confirm agents can install and launch the required browsers and operating-system dependencies. Decide whether traces or other failure artifacts are needed to diagnose problems.
- Choose for the environment. Browser coverage, remote execution, team conventions, and maintenance constraints matter more than a claimed universal “best” framework.
Common setup problems and fixes
| Symptom | Likely cause | What to check |
|---|---|---|
| pytest reports that no tests were found | File or function names do not match discovery conventions, or the command runs from the wrong directory. | Use test_*.py or *_test.py, ensure test functions start with test, and run pytest from the project root or provide the test path. |
A unittest test does not run under discovery |
The test does not follow the expected case and method conventions, or discovery starts from an unsuitable directory. | Check that the class subclasses unittest.TestCase, methods begin with test, and discovery runs from the intended project location. |
| pytest is unavailable in the shell | It may be installed outside the active environment. | Activate the project virtual environment and install pytest there; python -m pytest uses the selected interpreter. |
| A browser test cannot launch in CI | The agent may lack a browser, system dependencies, or a valid driver/session setup. | Follow the browser tool’s CI installation steps and verify the actual agent image. For Selenium remote sessions, check Grid availability; for Playwright, install browser dependencies as documented. |
| A browser remains open after a failing test | Session cleanup is missing or not registered as teardown. | Use fixture teardown or a cleanup hook that calls driver.quit() regardless of test outcome. |
| A UI test fails intermittently around loading or navigation | The page or dependent service may not be ready when the assertion runs. | Wait for the relevant application state rather than relying on arbitrary timing, and retain useful failure artifacts where the CI workflow supports them. |
Browser screenshots for test evidence
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Or skip the browser setup
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